Bacnet logoA protocol for building automation

BACnet is a data communication protocol mainly used in the building automation and HVAC industry (Heating Ventilation and Air-Conditioning).

The protocol allows equipment such as air conditioning machines, pumps and ventilation devices to communicate with a PLC. This creates "intelligent" houses with a high degree of automation.

BACnet stands for Building Automation and Control Networks and was first developed in 1987 with the backing of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). BACnet is an American national standard, a European standard, a national standard in more than 30 countries, and an ISO global standard.

What is communicated over BACnet?
BACnet specifies most all of the most common functions used in a building: analog and binary input, output, and values; control loops; schedules, etc.  

BACnet communicates by using requests towards "objects." An object is simply a collection of information related to a particular function that can be uniquely identified and accessed over a network in a standardized way.

BACnet defines 23 standard object types:
Bacnet objects  

Serial and Ethernet versions
There are a couple of different serial and Ethernet based-versions of BACnet. The most common serial version is called BACnet MS/TP while the dominant Ethernet version is BACnet/IP.

BACnet/IP has been developed to allow the BACnet protocol to use TCP/IP networks. You could say that BACnet/IP is a way of hooking BACnet up to the Internet and communicate with different Local Area Networks (LANs). This enables system owners, facility managers, or even external suppliers to access BACnet networks and manage their devices and systems remotely.

BACnet MS/TP facts
Network Type:    Serial RS-485 with a Master/Slave Token Passing protocol
Topology:    Line topology
Installation :    Any cable with at least 3 conductors can be used.
Distance between nodes depends on baud rate.
Speed   9600 kbit/s,19200 kbit/s, 38400 kbit/s and 76800 kbit/s
max. Stations:   

127 MS/TP masters

Data :   

Up to 480 bytes per telegram frame

Network Features :   Low cost, low speed network topology for BACnet.
User Organization:


BACnet/IP facts
Network Type:    Ethernet based network using UDP for data transfer.
Topology:    Line or star topology (Standard Ethernet topology)
Installation :    Ethernet twisted pair cables with RJ45 connectors
Speed   10/100 Mbit/s full duplex
max. Stations:   

No network limitation of number of nodes.

Data :   

Up to 1476 bytes per frame

Network Features :   BACnet over standard IP networks.
User Organization:

See Anybus products supporting BACnet